The present invention generally relates to a method and an apparatus for forming images on both sides of a recording sheet, and particularly to a method and an apparatus for successively forming images on both sides of different sized recording sheets. The method and the apparatus for forming images on both sides of the recording sheet of the present invention can be adapted for use with different image forming apparatus, such as laser beam printers, copy machines and facsimile machines.
There is a conventional method forming images on both sides of a recording sheet in the following manner. That is, a reversing unit and an image forming unit are provided in a transport path of a recording sheet. First, the image forming unit forms an image on a first side of the recording sheet which is supplied from a paper supplying unit. Then, the recording sheet having the image formed on the first side thereof is reversed by the reversing unit, and then the reversed recording sheet is supplied to the image forming unit. The image forming unit then forms an image on a second side of the recording sheet.
For example, when copying 60 pages of a document onto both sides of 30 recording sheets so that 30 duplex copies are obtained, 30 pages of the document are first copied onto the first sides of 30 recording sheets by the image forming unit. The 30 recording sheets each having an image formed on the first side thereof are then reversed by the reversing unit and then stacked on an intermediate tray. Thereafter, the remaining 30 pages of the document are copied on the second sides of the 30 recording sheets by successively refeeding the stacked recording sheets from the intermediate tray to the image recording unit. The duplex print mode in which the images are formed on both sides of the recording sheets in accordance with the above procedures is often referred to as a stack mode.
A conventional apparatus forms duplex prints in accordance with the stack mode described above.
According to the conventional stack mode, the recording sheets must be uniformly stacked on the intermediate tray to prevent each recording sheet refed from the intermediate tray to the image forming unit from being skewed. However, when images are formed on both sides of different sized recording sheets, it is difficult for the recording sheet to be uniformly stacked on the intermediate tray. For this reason, small sized recording sheets are skewed when refed from the intermediate tray to the image forming unit, so that a paper jam is generated and the quality of the image formed on the second side of the refed recording sheet deteriorates.
In addition, the recording sheets stacked on the intermediate tray must be separated one by one by a separating unit when the stacked recording sheets are successively being refed to the image forming unit. For this reason, when a malfunction of the separating unit occurs, there is a problem in that two or more recording sheets may be simultaneously refed from the intermediate tray to a image forming unit, so that the paper jam may occur.
In addition, when a paper jam occurs, it is difficult to obtain accurate information about the page which is not printed. Thus, there is a problem in that the operation of making the duplex prints in accordance with the stack mode must be repeated from the beginning.